D2D discovery consists of an application where user equipments (UEs) sense each other's presence based on detection of discovery messages periodically broadcasted by the UEs. Each UE typically transmits one or more discovery messages within configured resources (discovery resource pool). Discovery resource pools may be different across cells, and the discovery resource pools may be offset in time due also to synchronization differences between cells. This feature of discovery resource pools implies that UEs in one cell need to synchronize to the pools provided by other cells in order to detect the associated discovery messages. One way of providing this synchronization is to associate pools with synchronization signals that are particularly suitable for the purpose.
Typical resource allocation for discovery includes a discovery period that may be several seconds long. Within the discovery period, each discovery pool typically spans a small fraction of resources. The discovery resources should be almost contiguous in time for energy efficiency reasons. The resource pool may only span a few tens or hundreds of milliseconds within the discovery period.
D2D Synchronization Signal (D2DSS)
Traditional communication in terrestrial radio networks is via links between User Equipments (UEs) and base stations. However, when two UEs are in the vicinity of each other, direct device-to-device (D2D) communication may be considered. Such D2D communication may be dependent on synchronization information from (1) a base station, (2) a different node such as a cluster head (CH) (i.e., a UE acting as a synchronization source) providing local synchronization information, or (3) a UE enabled to relay synchronization information from a different synchronization source. An eNB/CH is used as the synchronization source for intra-cell/cluster communication. Relayed synchronization signals are used for inter-cell/cluster communication.
FIG. 1 is an illustrative illustration of a cellular network in which different nodes act as synchronization sources. Within Cell-1, an eNB 11 provides synchronization information to UEs 12-14. Within Cluster-1, UE 15 acts as a Cluster Head (CH) to provide synchronization information to UEs 16 and 17. Note that UE 14 in Cell-1 and UE 16 in Cluster-1 may also act as relays to send synchronization information between Cell-1 and Cluster-1. Within Cell-2, an eNB 18 provides synchronization information to UE 19. UEs 12, 13, and 19 may act as relays to exchange synchronization information between Cell-1 and Cell-2.
For in-coverage D2D scenarios, the synchronization reference is provided by the eNB. The eNB signals the D2D resource pool to indicate the resource used for D2D. For out-of-coverage D2D scenarios, the CH provides the synchronization reference.
The signal design of D2D synchronization signals (D2DSS) is under discussion in 3GPP. One proposed solution consists of differentiating D2DSS into two sets: one set intended to support D2D discovery and the other set intended to support D2D communication.
Synchronization signals are typically periodic. The reason is that oscillators tend to drift from their nominal value and periodic corrections based on received synchronization are needed to maintain relative synchronization. Typical synchronization periodicity may be from a few milliseconds to tens or hundreds of milliseconds.
During Long Term Evolution (LTE) Rel-12 standardization, a distributed synchronization protocol was defined for D2D, where devices derive their transmit synchronization reference either from an eNB (when in coverage), from another device (when out of coverage), or autonomously when they are isolated. Synchronization signals/channels transmitted by an in-coverage device include information that the original synchronization reference is from an eNB. Out of coverage devices that detect SLSS from an in-coverage device prioritize synchronization to such reference, under a number of radio conditions that are not relevant for this disclosure. As a consequence, devices that are just outside eNB coverage are likely using a transmit synchronization reference originally from the eNB, even though they are synchronized to another in-coverage device.